1. Field of the Invention
The invention relates to an electroabsorption modulator, a modulator laser device and a method for producing an electroabsorption modulator.
Semiconductor laser diodes which are used as transmitting elements in optical telecommunications must simultaneously fulfill a plurality of requirements which can, however, be optimized only in dependence on one another. For example, in the case of a direct modulation in a semiconductor laser diode, only a high current density or a high internal light intensity ensures a fast intrinsic modulability, but at the same time parasitic effects such as parasitic resistances, parasitic capacitances and parasitic inductances in the supply leads should be minimized, and the internal heating of the component should be limited. This can be achieved with the aid of an optical modulator driven separately electrically. Specifically in the case of a laser structure in which the resonator fixes the wavelength—for example in the case of a distributed feedback laser (DFB laser) or a vertical-cavity surface-emitting laser (VCSEL), the relative displacement of the laser wavelength and the absorption edge with temperature mostly ensures a narrow temperature window in which the modulation principle functions.
It is therefore desirable to have a modulator which can be used in a wide spectral and temperature range. Moreover, it is also desirable for the transmission of digital signals likewise to have a digital modulation principle in which the optical modulator can assume only two states, for example absorbing (“off” state) and poorly or non-absorbing (“on” state), and these states cannot be influenced by the preceding signal sequence.
If, for example, the active surface of the laser is reduced, a high current density and a fast modulability together with a limited thermal heating of the laser are achieved with small currents through the active surface. At the same time, however, the series resistance grows because of the current constriction. In conjunction with existing non-scalable capacitances at the connecting contacts (pads) and in the driver circuit, this leads to an undesired additional RC limitation of the modulability.
An external modulator is normally used, first and foremost, in telecommunications applications. However, this is expensive in the datacom sector and would precisely nullify the advantage of an inexpensive laser diode, for example a vertical emitter. By contrast, because of the required compactness, in the case of integrated modulators use is predominantly made of direct modulation of the imaginary part of the refractive index in so-called electroabsorption modulators.
2. Description of the Related Prior Art
Laser diodes with a monolithically integrated electroabsorption modulator are already known from the prior art, for example from [1], [2] or [3]. In this case, for example, the Quantum Confined Stark Effect, shortened below to QCSE, is utilized in order to displace the absorption edge in the modulator and thereby to switch the modulator to and from between the “off” state and the “on” state. With such a modulator it is only the efficiency of the charge carrier removal, that is to say the charge carrier emission from the quantum wells and the drift over the field region, which limits the intrinsic speed by analogy with photodetectors. It is disclosed in [4] in this context that filling effects and changes in the local electric field should be avoided because of their strong effects on the optical properties.
A substantial disadvantage of this modulation principle is, however, the limited effect of the displacement of the quantum well band gap or of the fundamental exciton absorption concerned as a function of the applied field. In the case of a typical VCSEL structure, which should be operated uncooled between 0° C. and 85° C., the relative displacement between maximum gain and emission energy is approximately 30 meV. Moreover, a deviation of up to ±10 meV between laser resonance and modulator band gap should also be permitted in order to be able to compensate layer thickness tolerances between the individual components.
In order in an appropriate modulator quantum well to adapt only the band gap or fundamental exciton absorption line by the overall amount set forth above in relation to the resonator wavelength, an absorption edge displacement of 50 meV will already need to be achieved via the change in bias. According to [5], the realistically achievable displacement is approximately half as large. The fields required for this purpose of the level of a few 105 V/m would lead to dissociation of the excitons, and the modulation characteristic would not be uniform in the overall operating range. In addition, for a given voltage range the large field region in the system limits the length of the intrinsic region and thus the minimization of the capacitance.
Moreover, nonlinear effects such as impact ionization are also to be considered. For GaAs, the ionization coefficient for electrons is just 104/cm at 250 kV/cm. Consequently, electroabsorption modulators which use the QCSE can be used without a problem only in temperature-controlled systems with defined detuning of the resonator wavelength on the one hand, and between the gain and absorption spectra of the active regions, on the other hand.
By contrast with the QCSE modulator, in the case of a modulator which operates with charge carrier filling, when it is switched into the transparent state, the charge carriers are firstly transported to the quantum well and then captured there. Consequently, in the case of this modulator type both the charge carrier emission process and the charge carrier capture process form the fundamental speed limitations. The charge carrier capture in quantum wells with good charge carrier inclusion proceeds yet more quickly than the charge carrier emission and is of the order of magnitude of 10−12 in accordance with [6].
Neither capture nor emission times would be a fundamental limitation for targeted modulation frequencies up to 40 GHz, since these can be kept shorter than 5 ps by means of a favorable quantum well design and, in the case of the emission time, by means of correspondingly high fields. However, this holds only as long as the charge carrier recombination which is slower by several orders of magnitude is not used for switching, and as long as the charge carrier transport by means of drift or diffusion is fast enough. In the case of a pin quantum well structure being forwardly polarized, transport on the undoped barriers at low carrier densities essentially only takes place by means of diffusion. A pin quantum well structure is to be understood in this case as a quantum well structure of a strongly doped p-region, a strongly doped n-region and an intrinsic region lying therebetween.
The diffusion time for holes is determined in accordance with τdiff=Li2/4Dh. In the case of an assumed spacing of the quantum well from the p-doped region of Li=100 nm and a diffusion constant at room temperature of Dh=kTμh/q=5 cm2/s for Al0.2Ga0.8As, this results in such a case in a transport time of approximately 5 ps, but this grows quadratically with the diffusion length. Depending on the quantum well design and doping profile, thus, it is either the transport time or the physical capture time which predominates.
If the undoped diffusion regions are reduced, the capacitance is increased, however. This has a disadvantageous effect on the modulation rate if the charge carriers need to be removed from the quantum well again not, as in the laser, already by means of stimulated recombination, but only by means of a change in the external voltage. In this case, the space charge capacitance in series with the bulk resistance leads to an RC limitation of the modulation bandwidth. The intrinsic series resistance is determined chiefly by the p-doped lead layer on the basis of the low hole mobility in semiconductor materials. Consequently, it would be desirable to have a concept which permits optimum setting of capacitance, transport times and bulk resistance depending on semiconductor material used, modulated design and parasiticities of the lead and/or drive. Thus, the bulk resistance can be substantially reduced, for example, when exclusively n-doped lead layers are used.
Such a modulation principle, which comprises nipin-structures (structures composed of a layer stack of n-doped layer, intrinsic layer, p-doped layer, intrinsic layer and n-doped layer) and operates chiefly with electron filling into a quantum well from a neighboring n-doped heterobarrier (reservoir), has become known under the designation BRAQWET (Barrier Reservoir And Quantum-Well Electron-Transfer) (compare [7]). In accordance with the BRAQWET, the so-called Burstein-Moss-effect is used, that is to say the reduction of the absorption by filling only one sort of charge carrier into the quantum well. Since the state density of the conduction band is normally substantially smaller than that of the valence band, the quantum well is filled with electrons.
Consequently, degeneracy is achieved as early as with a low charge carrier density of approximately 2×1018 cm−3, and absorption saturation in the region of the band edge, on the basis of the Pauli exclusion principle. The advantage is that the absorption profile can be displaced both in frequency by means of the QCSE, and also in amplitude by means of filling. Consequently, an increase in the field leads in both cases to increasing the absorption. The electron transport times are generally negligible. However, the structures have some disadvantages. Because of the need to optimize electron filling, operations should be conducted with sufficiently high diffusion barriers relative to the electron reservoir. In accordance with [8], this in turn limits the electron emission rate upon switching over to maximum absorption. The effective barrier height is lowered with high fields, if appropriate. Furthermore, it is known in accordance with the prior art to render the reservoir barrier continuous, it thereby being possible to shorten the electron emission times virtually at will. However, in principle this is done at the cost of the electron inclusion. However, the pump-probe measurements published in [9] exhibit no worsening in the electrooptical properties. In the case of optical excitation, however, long effective hole emission times were observed in the nanosecond region. The barrier height on the extraction side for holes is very high in BRAQWETs, in order to configure the electron filling efficiently and with a low leakage current. The negative effect of the field shielding of remaining holes is not yet explained in this case. In general, it is either possible for a given voltage shift to maximize the absorption shift into a larger spectral range, or to optimize rate. Furthermore, in the case of unipolar filling the state of transparency cannot be completely achieved, and the absorption shift is still a function of temperature, although the spectral dependence of the absorption shift is already reduced by contrast with pure QCSE modulators.
In addition, only a quantum well can be filled efficiently in unipolar fashion per npn region. Consequently, several absorption regions are mostly arranged one above another. In accordance with [7], this multiplies the voltage requirement. During a lengthy “on” state (absorption minimum in the modulator), by contrast, a state of transparency is achieved nevertheless because of the generation of holes on the basis of the non-vanishing absorption. On the one hand, the modulation depth is thereby a function of the bit sequence, while on the other hand the plasma then produced must be removed from the pn junction or the quantum well. This does lead, finally, to an increased space charge capacitance.
Consequently, the respective other charge carrier type, which necessarily arises upon absorption, should be efficiently swept out even in the case of a theoretically pure absorber operating in a unipolar fashion.
Disclosed in [10] is an optical electroabsorption modulator in which a first upper cladding layer and a second upper cladding layer are provided over an optical absorption layer. Provided between the first upper cladding layer and the second upper cladding layer is a barrier layer which is provided for the purpose of preventing a diffusion of foreign atoms from the second upper cladding layer or thereabove into the first upper cladding layer and the optical absorption layer.
A monolithically integrated laser diode modulator with a strongly coupled super-lattice is disclosed in [11]. In this laser diode modulator, the same epitaxial layer, specifically a strongly coupled, combined super-lattice, is used as active layer of the laser diode and as absorbing layer of the modulator.
[12] discloses an integrated modulator semiconductor laser device which is produced on a semiconductor wafer by means of selective crystal growth. For this purpose, each chip region on the semiconductor wafer is divided into two semiconductor regions. There is produced on each first semiconductor region a semiconductor laser which can emit laser light, and there is produced on each second semiconductor region a light modulator which can modulate the intensity of the laser light emitted by the semiconductor laser.
A semiconductor device with cascade-modulation-doped quantum well heterostructures is disclosed in [13]. In this semiconductor device, known modulation-doped quantum well heterostructures are cascaded in order to increase the rate of functioning without significantly increasing the operating potentials.
Moreover, [14] discloses a semiconductor device with polarization-independent stacked heterostructure, which is similar in its design to the semiconductor device known from [13].